For example, there is an unvulcanized rubber sheet for a conveyor belt as an unvulcanized-rubber formed object. This unvulcanized rubber sheet for a conveyor belt is generally vulcanized and molded using a vulcanizing belt press. During the process of vulcanizing, due to entrapped air, and due to gas, water and the like generated from the unvulcanized rubber material, clearances are produced between the heat plate of the vulcanizing belt press and the surface of the unvulcanized rubber sheet pressed by the heat plate, thereby forming depressions on the obtained conveyor belt surfaces. Therefore, the conveyor belt surface cannot present a substantially smooth and glossy surface and there is a problem of having a bad appearance. Particularly in these years, a good appearance is desired by users in addition to an excellent property.
To solve the above problem, there is conventionally employed a method of coating powder of, for example, zinc stearate, talc, mica or the like on the surface of the unvulcanized rubber sheet. By coating this powder, the entrapped air, and the gas and water (steam) generated from the rubber material (inside) between the heat plate and the unvulcanized rubber sheet at the time of vulcanizing can be exhausted to the outside via the clearance formed between the powder and the heat plate. Accordingly, the generation of depressions is restricted and the conveyor belt which improves appearance can be obtained.
However, in the method of coating powder as described above, due to the insufficient clearance between the unvulcanized rubber sheet and the heat plate, there is a problem that the air and gas cannot be exhausted effectively. Since the powder scatters when coated, there is also a problem of harmful influence on the working environment.
Further as the other method, in Japanese Patent No. 2588209, there is proposed a method for vulcanizing an unvulcanized rubber sheet that has a sheet-like sponge member adhered to the surface thereof, the sheet-like sponge member having a continuous clearance. During vulcanization by a vulcanizing belt press, the rubber being gradually softened enters into the clearance of the sponge member and the rubber pushes out the residue air and the like through the continuous clearance to the outside. Therefore, the generation of the residue air is prevented and depressions is not produced on the surface of the obtained conveyor belt
However, in the method using the sponge member, the sponge member is so excellent in expansion that, when adhered to the unvulcanized rubber sheet surface by pressing, the sponge member extends to the longitudinal direction and reduces with respect to the width direction. Therefore, when the sponge member is adhered to the unvulcanized rubber sheet surface, it is likely to generate a part that is lack of the sponge member in one or both end portions of the width direction of the unvulcanized rubber sheet surface. Accordingly, it is difficult to adhere the sponge member through the whole surface of the unvulcanized rubber sheet, and a problem of deteriorating press-processing efficiency is caused. Further, since the obtained conveyor belt has a structure such that the sponge member having a low strength is disposed on the surface, the abrasion resistance of the surface is deteriorated.